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  #21  
Old 14-11-2018, 09:50 AM
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Peter Ward
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Quote:
Originally Posted by bojan View Post
I think this is not quite the case - by addiing lenses, they are essentially adding the frames to the stack.

The only thing saved here is time (but not money, considering the price of Canon 400mm f/2.8 lens)
Well, yes thatís correct, the resolution of the system it still limited by that of a single lens. I was really alluding to its light gathering capacity...which inreases as lenses are added.
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  #22  
Old 14-11-2018, 10:11 AM
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Stellar limiting magnitudes are totally different from faint nebulae and/ or extended objects.
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  #23  
Old 14-11-2018, 10:27 AM
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Stellar limiting magnitudes are totally different from faint nebulae and/ or extended objects.
I'm glad someone else also noticed that elephant in the room
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  #24  
Old 14-11-2018, 07:36 PM
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Atmos (Colin)
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This whole astrophotography thing is quite complicated in one sense but in another sense it really isn't. One of the brilliant things about mathematics is that as long as you're consistent you can do whatever the hell you want! Let me explain.

When I am comparing optical systems to get a sense of SNR and integration times I calculate a rudimentary flux term for comparisons which is telescope aperture/resolution. Take the FSQ106 KAF-16803 as an example.

106^2*((9*206.265)/530)^2 which gives me a value of 137,847 (I'm rounding to whole numbers).

Now let's compare that to a system that has near identical image scale but done at 5% of the cost!
QHY183M + Samyang 135mm F/2 @ F/2.8
48.2^2*((2.4*206.265)/135)^2 = 31239

They both have very similar FOV, very similar image scale of 3.5"/pixel (FSQ) and 3.67"/pixel but the FSQ is 4.4x higher SNR for the same integration; 5 hours with the FSQ would require 22 hours with the Samyang for the same SNR. This SNR calculation is purely for extended sources like dusty regions as point light sources are purely a function of aperture where the FSQ captures 4.8x the amount of light.

With the Dragonfly array it depends on how you look at it. One one hand you can say that an increase of 30 to 48 400mm F/2.8 lens' is taking you from a 782mm F/0.51 to a 990mm F/0.4 OR you can think of it as being able to capture 180 hours per night to 288 hours per night (taking 6 hours as being your imaging time on a spot of sky at a time) with a lens. It all depends on how you look at it.

When it comes to professional observatories and large apertures, you're generally chasing down significantly fainter objects where capturing large amount of photons does become important. Take what Suavi mentioned earlier when he said he was capturing as little as 1 photon per hour in some patches around the Helix. Comparing this to Keck and its 10m mirror which would be capturing 2.52 photons per second (9070 per hour) from that same patch of sky, you begin to understand why bigger can be better! Now Keck is imaging at a much higher image scale than Suavi so those photons do get spread out a lot more amongst the pixels so it may end up working out to more like 300 photons per hour per pixel but with the added resolution that goes with the increase in image scale.

EDIT:
I've had it pointed out that my figures don't quite match what they should and I've figured out that I should be multiplying the aperture and not dividing it.

Last edited by Atmos; 18-11-2018 at 12:15 PM.
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  #25  
Old 21-11-2018, 08:59 AM
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For any system the really dim stuff is in the bottom few digits of your sixteen bit camera.

This is after calibrating for darks and bias. This also should remove the floor.

The approach I take is to process all corrected images in 32 bit format so the dim stuff is spread over far many more binary digits. This has the advantage when stacking many frames so that the dim stuff has more inherent dynamic range and it lowers any chance of posterisation when stretching.

I have come to the conclusion that I can do better by binning x2 to have effectively 18 micron pixels with my FPL16803 and F3 600mm RH200 Optic. The read noise is also lowered by a factor of four when binning x2.

I get resolution back by doing drizzle integration. The more subs you have the better the resolution as well as the signal to noise.

I download images at the lowest speed of 1MHz and use 100 Bias and Dark frames for correction. It is pointless to introduce systemic noise at the correction phase. The camera is also flushed after each exposure. You would be surprised what is left in the wells from bright stars.

When correcting for darks I always use darks collected at the same temperature as the lights. Even PixInsight's algorithm fails dismally for temperature differences for darks and lights for really dim stuff.

Dithering is the other thing to do routinely.

Remember folks that collecting star images are limited by diffraction for a given aperture and collecting very dim extended objects by focal ratio.

Of course aperture is king and always wins.

When they make a 600mm F1.5 optic I will get one. By binning x2 my system passes for this given I collect enough subs.

I welcome any critique of my thinking. I hope this helps others.

Bert


See this for an example.

http://www.iceinspace.com.au/forum/s...d.php?t=171515

Last edited by avandonk; 21-11-2018 at 11:44 AM.
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  #26  
Old 21-11-2018, 09:15 AM
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Hi Bert,

Would your subs be read noise limited if they were unbinned?

Have you measured how much read noise advantage you get from binning the PL16803? In the case of my U16M (also a KAF-16803 sensor) the read noise in a 2x2 binned pixel is not much less than four times the read noise in a single pixel. I have heard similar comments about KAF sensors in general from other folks who have done the measurements.

There are disadvantages to binning then drizzling so I'd be interested to know if you did an analysis to show that it was worthwhile.

Cheers,
Rick.
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  #27  
Old 21-11-2018, 09:30 AM
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Quote:
Originally Posted by avandonk View Post

...

Remember folks that collecting star images are limited by diffraction for a given aperture and collecting very dim extended objects by focal ratio.

Of course aperture is king and always wins.

When they make a 600mm F1.5 optic I will get one. By binning x2 my system passes for this given I collect enough subs.

...
Well you can always DIY-it (in terms of light gathering) with four 600mm f/3 optics to get that (Well almost. It would be more like f/1.8 in light gathering terms, accounting for the secondary obstruction). It might cost a little more

Best
JA
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  #28  
Old 21-11-2018, 10:20 AM
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Quote:
Originally Posted by RickS View Post
Hi Bert,

Would your subs be read noise limited if they were unbinned?

Have you measured how much read noise advantage you get from binning the PL16803? In the case of my U16M (also a KAF-16803 sensor) the read noise in a 2x2 binned pixel is not much less than four times the read noise in a single pixel. I have heard similar comments about KAF sensors in general from other folks who have done the measurements.

There are disadvantages to binning then drizzling so I'd be interested to know if you did an analysis to show that it was worthwhile.

Cheers,
Rick.
To get the same sort of signal to noise at full resolution I needed at least fifty images for the dim stuff.

I have done the experiment and compared images. Binning and drizzling wins.

I must admit I am getting better at using PixInsight so it could be an aberration.

So I went back to some previous data and reprocessed it using the new protocols.

I have not measured the gain in signal to noise by binning by any objective means apart from image quality and it works.

I have decided to use my laboratory quality power supply to feed my camera as the supplied switch mode power supply has died.

I worked out what pins go where and am in the process of making up a lead.

It may not make a difference. Time will tell.

There is only one absolute in science and it is signal to noise ratio.

Bert
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  #29  
Old 21-11-2018, 11:32 AM
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Quote:
Originally Posted by avandonk View Post
For any system the really dim stuff is in the bottom few digits of your sixteen bit camera.
[...]
I have come to the conclusion that I can do better by binning x2 to have effectively 18 micron pixels with my PL1803 and F3 600mm RH200 Optic. The read noise is also lowered by a factor of four when binning x2.

Is that a CMOS or CCD? (couldn't find a PL1803 anywhere on Google) I understand CMOS cameras don't really benefit from hardware binning, but maybe I should rethink that advice...

Best

Markus
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  #30  
Old 21-11-2018, 11:38 AM
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Quote:
Originally Posted by Stonius View Post
Is that a CMOS or CCD? (couldn't find a PL1803 anywhere on Google) I understand CMOS cameras don't really benefit from hardware binning, but maybe I should rethink that advice...

Best

Markus
He did mean the FLI PL16803 but missed a digit
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  #31  
Old 21-11-2018, 11:44 AM
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He did mean the FLI PL16803 but missed a digit

Fixed! Bert
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  #32  
Old 21-11-2018, 04:17 PM
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There is only one absolute in science and it is signal to noise ratio.
Which you're measuring subjectively
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  #33  
Old 21-11-2018, 04:29 PM
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I thought the only absolute in science was the speed of light in vacuum?
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  #34  
Old 21-11-2018, 04:42 PM
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I can go full blonde - that's pretty dim...

OK, jokes over, back to science
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  #35  
Old 23-11-2018, 02:08 PM
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May be we should re-read this:
https://www.cloudynights.com/article...t-part-1-r1895
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